The present disclosure relates to a radiation tomography apparatus which captures a tomographic image of a subject and more particularly relates to a radiation tomography apparatus which can capture both an anatomical image and a functional image and display these images while superimposing one upon the other.
Positron emission tomography (PET) devices have been developed to capture an image representing the distribution of a radiopharmaceutical administered to a subject. A PET image captured by these PET devices is a tomographic image of the subject, and more specifically, a functional image representing an internal activity of the subject. Specifically, if a circular object is shot on a PET image, that circular part indicates the state of a particular region of the subject in which a radiopharmaceutical is distributed at a high concentration. However, the circular object shot in the PET image does not necessarily mean that the subject has a structure of the same shape as that object. In other words, the PET image is not an image representing the exact shape of, for example, an organ of the subject.
Such a state of the art poses a problem when a diagnosis is made with reference to a PET image. Specifically it is impossible, just by looking at the PET image, to accurately determine which organ of the subject has the radiopharmaceuticals at such high concentrations and exactly what part of the organ is the region of interest.
Hence, there has been proposed a technique for displaying a CT image (an MR image) and a PET image while superimposing them one upon the other. A CT image, captured with an x-ray, is called an “anatomical image.” In other words, if a circular object is shot on a CT image, it means that a structure having the same shape as the circular object exists in the subject. Suppose the subject is virtually sectioned on a transverse plane of the subject, the section becomes the same as the CT image as viewed on that transverse plane. When the PET image and the CT image are displayed while being superimposed one upon the other, the superimposed image tells the user exactly which organ of the subject has the radiopharmaceuticals at such high concentrations and precisely what part of the organ is the region of interest.
The PET image and the CT image cannot be captured by the same device. Thus, it is important to accurately position these two images with respect to each other before superimposing them on upon the other. That is to say, misalignment will occur between the PET image and the CT image unless the transverse plane of the PET image is the same as the transverse plane of the CT image to be superimposed on this PET image. Such misalignment needs to be avoided to make an accurate diagnosis.
In a device that displays a PET image and a CT image while superposing them one upon the other, a PET device and a CT device are integrated together. In such a device, the positional relationship between these devices is known in advance. Thus, it is easy to capture a CT image having the same transverse plane as a PET image. The transverse planes of the CT image and PET image captured by such a device are merely spaced apart from each other by the distance between the two devices.
Another technique for displaying a CT image superimposed on a PET image includes utilizing a PET device and a CT device which are provided in two different rooms. According to such a technique, both of the devices previously execute calibration imaging in order to make correction to a sectioning position. When the images are superimposed one upon the other, the result of the calibration imaging is used to correct the imaging positions to avoid the occurrence of misalignment between the images (see International Patent Application No. WO2007/03730, for example).
However, the conventional radiation tomography apparatus has the following problem. Specifically, the conventional radiation tomography apparatus is not configured such that one of the two devices is movable with respect to the other device.
Currently, a radiation tomography apparatus is being developed which is configured such that one of a PET device and a CT device is fixed and the other is movable. Such a configuration allows for not only recombining and removing the devices according to the purpose of an inspection but also a more flexible application of the radiation tomography apparatus.
It is difficult for such a partially movable radiation tomography apparatus to capture a CT image having the same transverse plane as a PET image, and thus misalignment occurs easily between the two images. The misalignment tends to be caused due to an inconsistent distance between the two devices. This is a difference between the partially movable radiation tomography apparatus and the radiation tomography apparatus in which the CT device and the PET device are integrated together.
Moreover, it is impossible to apply as it is, to the partially movable radiation tomography apparatus, the conventional technique for correcting an imaging position between the PET device and CT device that are provided in two different rooms. In this case, the correction technique cannot be used unless the PET device and the CT device have different beds. The calibration imaging is executed with a marker placed on the bed. Here, the positional relationship between the bed and the imaging field of view of the PET device is known, and the positional relationship between the bed and the imaging field of view of the CT device is also known. In the partially movable radiation tomography apparatus, however, the positional relationship between the movable CT device and its bed is indefinite, even if the positional relationship between, for example, the PET device and the bed that are both fixed on the floor is known. Hence, a change in the positional relationship between the CT device and the PET device makes it impossible to use the correction technique involving the calibration imaging.
In view of the foregoing background, the present disclosure provides a partially movable radiation tomography apparatus which can superimpose an anatomical image and a functional image one upon the other without causing misalignment between them.